Question

I found boost has a class called context which is used for context switching, right?

I try to Google it but did not found any document or example. I am just wondering if anyone can provide some information.

Answer 1

Boost::Context is an official part of Boost in version 1.51.0 and up. See http://www.boost.org/doc/libs/1_51_0/libs/context/doc/html/index.html for information about it. Unfortunately the documentation is slightly different than the implementation, and some things have changed in SVN, so you’ll need to read the header files a little bit.

Here is an example I wrote the other day showing Boost::Context to make simple coroutines using Boost 1.51.0+latest SVN:

#include <array>
#include <functional>

#include <boost/context/all.hpp>

class Coroutine {
    public:
    Coroutine() :
        my_context(boost::context::make_fcontext(
            stack.data() + stack.size(),
            stack.size(),
            Coroutine::dispatch
        ))
    {}
    virtual ~Coroutine() {}

    void operator()() {
        boost::context::jump_fcontext(&yield_context, my_context, reinterpret_cast<intptr_t>(this));
    }

    protected:
    void yield() {
        boost::context::jump_fcontext(my_context, &yield_context, 0);
    }

    virtual void call() = 0;

    private:
    static void dispatch(intptr_t coroutine_ptr) {
        Coroutine *coroutine = reinterpret_cast<Coroutine *>(coroutine_ptr);
        coroutine->call();
        while (true) coroutine->yield();
    }

    private:
    boost::context::fcontext_t *my_context;
    boost::context::fcontext_t yield_context;
    std::array<intptr_t, 64*1024> stack;
};

struct A : public Coroutine {
    void call() {
        std::cerr << "A went to the store one day.\n";
        yield();
        std::cerr << "A was looking for groceries.\n";
        yield();
        std::cerr << "A finally found what she was looking for.\n";
    }
};

struct B : public Coroutine {
    void call() {
        std::cerr << "B went to the store one day.\n";
        yield();
        std::cerr << "B was looking for replacement tires.\n";
        yield();
        std::cerr << "B didn't find anything at all.\n";
        yield();
        std::cerr << "B went to another store.\n";
        yield();
        std::cerr << "B got the tires installed there.\n";
    }
};

struct C : public Coroutine {
    void call() {
        std::cerr << "C went to the store one day.\n";
        yield();
        std::cerr << "C was looking for USB drives.\n";
        yield();
        std::cerr << "C found several with competitive pricing.\n";
        yield();
        std::cerr << "C couldn't decide which to buy, so gave up.\n";
    }
};


int main() {
    std::cerr << "So, this is what happened.\n";
    A a;
    B b;
    C c;
    for (size_t i=0; i<10; ++i) {
        a();
        b();
        c();
    }
    std::cerr << "Then it all was done.\n";
}

Then compiling and running looks like this:

$ g++ -std=c++11 -o coroutines coroutines.c++ -lboost_context
$ ./coroutines
So, this is what happened.
A went to the store one day.
B went to the store one day.
C went to the store one day.
A was looking for groceries.
B was looking for replacement tires.
C was looking for USB drives.
A finally found what she was looking for.
B didn't find anything at all.
C found several with competitive pricing.
B went to another store.
C couldn't decide which to buy, so gave up.
B got the tires installed there.
Then it all was done.